1. Field of the Invention
The present invention generally relates to foils and more particularly to an improved method and apparatus for making high quality pore-free copper foil which is readily bondable to plastic substrates.
2. Description of Prior Art
Copper foil has been produced for may years by electrodeposition, the major use for such foil being as roof flashing in the housing industry. Of more importance, however, is the widespread use of electrodeposited copper foil in copper clad plastic laminates for use in the manufacture of printed circuits. For this latter application, the thickness of the copper foil is generally less while the copper foil is required to be free from porosity and of higher purity. As the requirements for printed circuits become increasingly severe because of greater design sophistication, so have the requirements imposed upon the copper foil used in the manufacture of the printed circuit laminates.
Early manufacture of copper foil by electrodeposition was performed on a rotating chromium plated steel drum, in various processes, such are described by Brown in U.S. Pat. No. 2,304,253, issued June 4, 1940. Because of the failure of such processes to control the nature of the oxide surface of the chromium plating on the drum, the copper foil produced on such drums was frequently quite porous and spongy and, therefore, unsatisfactory. Some years later, a slowly rotating drum having a lead surface was used in place of the chromium plated drum.
During the plating process on such a modified drum a portion of the lead surface of the drum was continuously polished by grinding it so as to provide a fresh plating surface for the electrodeposition of the copper foil thereon. This procedure, however, produced copper foil that frequently had fine lead particles from the surface of the drum trapped therein. When used to make printed circuits, such foil had many shorts between copper conducting lements because of the entrapped lead and therefore also was unsatisfactory.
U.S. Pat. No. 3,660,190 to Stroszynski, issued May 2, 1972, describes a procedure for manufacturing a composite material comprising a supporting film or foil and a metal layer bonded thereto. Basically, Stroszynski has combined the art of two conventional manufacturing processes; namely, electrolytic Cu foil formation and roll lamination. His process provides for electrodeposition of a copper layer on a first chrome plated roller and the subsequent transfer to a supporting film carried by an endless rotating intermediate support member, which can take the form of an endless belt on a guide roller or simply a second roller.
The drum surface of the guide or second roller, (as the case may be) performs the function of a pinch roller of the laminating process specifically recited and claimed.
Since the drum surface used for the preparation of the Cu foil is continuously reused as it rotates, it cannot function on a practical and efficient level. In the making of foil, having the specified thickness of 40 to 280 Microinches, it is difficult if not impossible to avoid the occurrence of flaws therein. Due to the presence of such flaws, resin and/or adhesive will be squeezed therethrough onto the drum surface, which will become progressively loaded with such deposits. On subsequent turns, the resin spots on the drum will inhibit copper deposition over ever increasing larger areas, thereby rendering an unacceptable film. Additionally, most resins will also contaminate the copper plating solution used for the electrodeposition. The key concept of Stroszynski is the dual function and reusability of his "endless rotatable surface", which is used as the intermediate support. Applicant is not concerned with this lamination function, but desires to provide a superior foil product free from the above noted flaws.
U.S. Pat. No. 3,151,048 to Conley, issued Sept. 1964, describes an improved procedure for making copper foil on chromium plated drums. The drum used in that procedure is capable of producing relatively pore-free copper foil. However, the process must be stopped and the drum must be periodically removed for expensive time-consuming regrinding and/or replating with chromium before it is reusable. Therefore, such process does not lend itself to long-term continuous production of high quality copper foil.
In the period from 1940 to the present, there have been many other attempts to improve, not only the drum material, but other parts of the apparatus, mostly with little success. The need has been recognized to overcome basic drawbacks inherent in the drum configuration in order to produce more economically a stringently controlled high quality copper foil. Thus, the electroplating current has been maximized in order to produce copper foil at a reasonable rate, since the foil is produced only on a portion of the surface of the drum, and the drum surface is relatively small. The total desired foil thickness so produced must be regulated by controlling the drums rotating speed.
After the desired copper foil thickness is electroformed, the foil is peeled from the drum surface and is wound onto a roll for further treatment, since the drum surface is needed in the manufacture of further copper foil. That surface of the copper foil so formed away from the drum, i.e. the side away from the drum surface, is quite rough and is usually treated further in a separate operation away from the drum in order to impart to it microscopic projections which enhance subsequent bonding of the copper foil to plastic substrates in conventional laminating processes.
The treatment applied to the rough surface of the copper foil to enhance its bondability is performed in a separate treatment machine and as a separate operation with a constant copper foil speed through the treatment machine, in contrast to the copper foil speed during its drum formation, which varies widely because of the different final copper foil thicknesses required. Therefore, the copper foil formation operation and the subsequent surface treatment, thereof, could not be combined into a single operation.
Accordingly, there is still a need for an improved method and apparatus for the production of high quality copper foil for printed circuits, which foil can be readily bonded to plastic. Such method should be capable of being operated at a constant speed and of effectively combining copper foil formation and surface treatment thereof. Such method should also be capable of maximizing the yield of pore-free copper foil suitable for laminates to be used in the maufacture of printed circuits. Preferably, such method should be capable of being used in a continuous long-term mode without requiring shut-down or slow-down and without any substantial variation in product quality.
Conventional processes, as noted above, require frequent shut-down of the apparatus to renovate the drum surface by remachining and/or replating, which raises the cost of the product. Such renovating, however, is necessary in order to avert substantial damage to the operating drum and the production of inferior product. Moreover, anodes of the nonconsumable type are employed in present apparatus, which anodes are shaped to closely follow the contour of the drum. Copper-rich plating solution is fed between the drum and anode surfaces causing erosion of the anodes thus requiring regular shut-downs and disassembly of the apparatus for anode replacement.
The desired method also should consistently yield copper foil of the highest pore-free quality. Current processes, in order to achieve good production economics and to attain reasonable foil production speed from the plateable drum surface, employ current densities usually very close to the maximum permissible for the equipment and plating conditions. The result often is unsuitable copper foil, thereby lowering overall production yields.